This invention relates to the manufacture of blind rivets. It relates more particularly to a method and means for making blind rivets of the threaded-mandril type.
The blind rivets with which we are concerned here comprise a generally cylindrical flange body shaped more or less like a stove pipe hat. The top of the rivet body may be open or closed depending upon the particular application. Extending into the rivet body from its flanged end is an elongated stem or mandril. The internal end of the mandril is threaded and is staked in position by radially upsetting portions of the rivet body into engagement with the mandril threads while simultaneously forming, intermediate the upset portions, a plurality of circumaxially spaced, longitudinally extending external ribs on the body.
The rivet is installed by inserting the rivet ribbed-end-first through registering openings in the parts to be connected together. Then the rivet is set using a tool which retracts the exposed end of the mandril while reacting against the flanged end of the rivet body thereby causing lengthwise contraction and radial enlargement of the rivet body between its ribbed end and the adjacent part. The enlargement reaches a sufficient size to retain the rivet body in the openings prior to the forceful disengagement of the mandril from the rivet body.
In the conventional manufacture of blind rivets of this type, fully described in U.S. Pat. No. 3,463,046, the threaded end of the mandril is inserted into the rivet body. The rivet body containing the mandril is then thrust closed end first between the relaxed jaws of a three-jaw split collet. The collet is then closed on the inner end portion of the rivet body to crimp it. The collet jaws are provided with arcuate circumaxially space, rivet-engaging inner faces which reshape the rivet body by forming circumaxially spaced, longitudinally extending deformations at the inner end of the body which are pressed into the mandril threads. These deformations are separated by circumaxially spaced, longitudinally extending ribs.
The assembly of rivets using the split collet arrangement results in extrusion of the rivet ribs into the gaps between the collet jaws so that the ribs project radially outward appreciably beyond the remainder of the rivet body. Consequently, in order to properly size the ribs as to their radial extent so that the rivet will fit in a standard size hole for that size rivet, a slidable pin must be provided which projects into the collet and pushes the entire rivet assembly back through a sizing die thereby forcing excess rib material back on itself so that the ribs no longer extend beyond the remainder of the rivet body. Thus the fabrication of each rivet requires not only a crimping operation but also a subsequent sizing operation.
The assembly of these rivets is slow also because the mandril-containing rivet body must be inserted into and ejected from the three-jaw collet and sizing die along the axis of the collet and die. Consequently, each crimped rivet assembly must be completely ejected from the collet and die before the insertion of the next rivet body-mandril pair can commence.
Consequently, the conventional apparatus for making these rivets is relatively complex and expensive. Moreover, being a two-step machine, it has a relatively low rivet production rate and a correspondingly high per unit cost.